1. Field of the Invention
The present invention relates to a servo circuit used for a video tape recorder (VTR), and particularly relates to a digital capstan servo circuit.
2. Description of the Prior Art
In general, a capstan servo circuit for a VTR is formed of a rotation speed servo system and a rotation phase servo system. Of the rotation phase servo systems used for the VTR, particularly the rotation phase servo system in a reproducing mode is controlled in such a manner that a control pulse (CTL) having the frequency of 30 Hz recorded on tracks of a tape is reproduced and then this control pulse becomes coincident with an inner reference signal. Although such servo technique is disclosed in detail in, for example, U.S. Pat. No. 4,242,619, such servo technique will hereinafter be described briefly.
FIG. 1 is a block diagram showing an example of a conventional digital capstan servo circuit including a rotation speed servo in the recording system when such rotation phase servo is performed.
In the figure, reference numeral 10S generally designates a rotation speed servo system and reference numeral 10P a rotation phase servo system. A rotation signal FG proportional to the revolution speed of a capstan is generated from a frequency generator mounted to a flywheel of the capstan though not shown. The rotation signal FG is supplied through a terminal 1 to a gate circuit 2 for a clock signal CK.sub.1 and the gate time for the clock signal CK.sub.1 is controlled in response to the rotation speed of the capstan.
The clock signal CK.sub.1 thus gated through the gate circuit 2 is supplied to a counter 3 which measures the rotation speed of the capstan and in which a counter output corresponding to the rotation speed of the capstan is formed. This counter output is supplied to a PWM (pulse width modulation) signal generator 4 as a PWM signal and a PWM output proportional to the rotation speed of the capstan is formed by the PWM signal generator 4. The PWM output is smoothed by a low pass filter 5 and then supplied through an amplifier or driver circuit 6 to a capstan motor 7 as a rotation speed control signal.
On the other hand, the rotation phase servo 10P is a constant phase servo which performs such a servo that the rotation phase of the capstan is locked to the phase of the inner reference signal.
As is known well, the capstan is provided with a pulse generator (not shown) from which a pulse signal PG indicative of the rotation phase of the capstan is generated. This pulse signal PG is supplied through a terminal 11 to a flip-flop 12 which will generate a gate pulse. And, an output from a reference signal oscillator 13 is supplied to a frequency divider 14 which then generates a reference signal PR having the frequency same as that of the pulse signal PG and a reference phase. This reference signal PR is supplied to the flip-flop 12 which thus generates a gate pulse corresponding to a phase difference between the signals PR and PG.
Consequently, an AND gate 16 delivers a clock CK.sub.2 during only the period in which the above gate pulse is supplied thereto. The clock CK.sub.2 delivered from the AND gate 16 is fed to a rotation phase measuring counter 17 to drive it. The counter output is supplied to a PWM signal generator 18 as a PWM signal, thus forming a PWM output corresponding to the rotation phase.
This PWM output is smoothed by a low pass filter 19 and then supplied to the capstan motor 7 as a rotation phase control signal similarly as mentioned above. Thus, the phase servo operation is performed in such a way that the rotation phase of the capstan is locked to the phase of the reference signal PR.
Reference numeral 20 designates an adder or mixer which adds the rotation phase control signal and the rotation speed control signal together.
As set forth above, according to the conventional capstan servo circuit, the rotation speed servo system 10S and the rotation phase servo system 10P are wholly formed independently.
Recently, such a phase servo system is proposed in which a pilot signal being frequency-multiplexed on a video signal is reproduced and this reproduced pilot signal is used as a reference signal for the tracking in a reproducing mode to control the revolution speed of the capstan motor to thereby perform the rotation phase servo.
First to fourth pilot signals S.sub.P1 to SP.sub.4 of single frequency (shown in FIG. 2), which are each constant in frequency interval and whose frequencies become high sequentially, are used as the above pilot signal. In order to record one pilot signal on one track being recorded, the first to fourth pilot signals S.sub.P1 to S.sub.P4 are sequentially frequency-multiplexed on the video signal and then recorded thereon at every fields. Thus, as shown in FIG. 2, frequencies f.sub.1 to f.sub.4 of the pilot signals S.sub.P1 to S.sub.P4 recorded on the tracks T.sub.1 to T.sub.4 which adjoin to one other become different from one other.
A track width T.sub.p in a reproducing mode is wider than a track width T.sub.R in a recording mode (see FIG. 3). Then, if as shown in FIG. 3 the crosstalk component of the pilot signals S.sub.P1 and S.sub.P3 from the adjoining tracks T.sub.1 and T.sub.3 upon playback mode is detected and the rotation speed of the capstan motor is controlled in such a manner that the levels of the pilot signals S.sub.P1 and S.sub.P3 may become equal to each other, the reproducing tracking can be established and thereby the rotation phase servo system of the capstan in the reproducing mode can be realized.